UV unwrap "Correct Aspect" stop functioning in Cycle mode
add a material with an image texture node


in cycles, the uv unwrap "correct aspect" feature has a hidden requirement


your mesh must have a material assigned
the material must have an "image texture" shader node
select your image texture, with the funny aspect ratio, in the node
now cycles can find your image's aspect ratio
uv unwrap's "correct aspect" works again



it's a bummer that there isn't a notice in place, for the "correct aspect" feature..
something like warning: "unwrap correct aspect" cannot find image texture node

also, i found that adding a "texture image node" helped when i couldn't get texture painting to work — it was ultimately the same kind of problem